chutney: on failure, explain which connection failed

When there's a failure in the chutney data transmission tests, it would be really, really helpful to know which connection failed. The debug flag doesn't quite do this yet, but it's pretty close.

This is particularly important now that chutney supports three different types of connectivity tests (in the same network):

client -> exit
bridge client -> exit
[bridge] client -> hidden service

Over two different protocols:

IPv4
IPv6 (currently bridges only)

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